The present invention relates to a photodetector and a unit mounted with a photodetector, and in particular relates to a photodetector that selectively receives signal light of a long wavelength (on the long wavelength side) when there is a plurality of incident signal lights of different wavelengths.
Currently there are broad applications for PIN photodiodes of a compound semiconductor material as photodetectors for optical fiber communications.
These PIN photodiodes employ a window structure to increase light-receiving sensitivity. In these PIN photodiodes, a light absorbing layer with a small forbidden bandwidth (long absorption edge wavelength) is formed on a semiconductor substrate, and a filter layer (window layer) with a large forbidden bandwidth (short absorption edge wavelength) is formed thereon. This structure makes it possible for light of the absorption edge wavelengths of the light absorption layer and the filter layer to be efficiently received.
A typical PIN photodiode is made of InGaAs/InP, where InGaAs is the material for the light absorption layer and InP is the material for the filter layer. In this case, the signal light in the wavelength range of 0.9 μm to 1.65 μm, which are the absorption edge wavelengths of InP and InGaAs, respectively, can be received with high sensitivity. An example of such a PIN photodiode structure is disclosed in Japanese Laid-Open Patent Publication No. 1-238070.
Hereinafter, a conventional PIN photodiode will be described with reference to FIG. 10. FIG. 10 is a cross-sectional view of a conventional PIN photodiode.
As shown in FIG. 10, an n−-InP carrier blocking layer (buffer layer) 102, an n−-InGaAs light absorption layer 103, an n−-AlAsSb carrier blocking layer 104, and an n−-InP filter layer 105 are sequentially laminated onto an n+-InP substrate 101. Here, the forbidden bandwidth of the AlAsSb is larger than that of the InP, and has an absorption edge wavelength of 0.67 μm.
A p+ diffusion region 106 made by the diffusion of Zn is formed in the filter layer 105 and the carrier blocking layer 104, and a surface protection film 107 is formed thereon. Additionally, a ringed anode electrode 108 is formed on the diffusion region 106, and a cathode electrode 109 is formed on the back face of the substrate 101.
It should be noted that since the PIN photodiode shown in FIG. 10 includes the carrier blocking layer 104, it has an improved sensitivity with respect to signal light of wavelengths of 0.9 μm or less over a PIN photodiode in which the carrier blocking layer 104 is not formed.
To explain the reason for this, signal light of wavelengths of 0.9 μm or less are absorbed by the filter layer 105 and generate electron-hole pairs, and in the case of PIN photodiodes in which the carrier blocking layer 104 is not formed, some of the electrons generated in the filter layer 105 flow into the light absorption layer 103 to become photoelectric current. In contrast, when there is the carrier blocking layer 104, the electrons are blocked by the hetero barrier at the interface between the filter layer 105 and the carrier blocking layer 104, and therefore do not become photoelectric current.
Passband type PIN photodiodes that are sensitive only to light of a narrower wavelength range have also been developed. For example, when performing wavelength multiplex communication with signal light of a 1.3 μm wavelength and signal light of a 1.55 μm wavelength, a passband type PIN photodiode sensitive only to those wavelengths is useful.
Here a PIN photodiode with short wavelength passband properties that is adequately sensitive to signal light of a 1.3 μm wavelength but has almost no sensitivity to signal light of a 1.55 μm wavelength can be readily achieved by forming the light absorption layer of the above PIN photodiode made of InGaAs, using an InGaAsP with an absorption edge wavelength of 1.4 μm.
On the other hand, long wavelength passband properties where there is sufficient sensitivity to signal light of a 1.55 μm wavelength but almost no sensitivity to 1.3 μm wavelength signal light can be attained by taking InP as the material for the carrier blocking layer 104 and InGaAsP with an absorption edge wavelength of 1.4 μm as the material for the filter layer 105. The relationship of the forbidden bandwidth of the carrier blocking layer 104 being larger than that of the filter layer 105 is maintained.
However, in conventional PIN photodiodes having passband properties, the filter layer 105 had to be made thick to increase the sensitivity difference with respect to long wavelength side signal light and short wavelength side signal light.
Accordingly, to achieve a PIN photodiode with long wavelength passband properties the material for the filter layer 105 should be InGaAsP, as described above. However, there is the problem that epitaxially growing a thick InGaAsP easily causes fluctuations in the composition of the InGaAsP.
On the other hand, in a planar PIN photodiode, in which impurities such as Zn are diffused in an island in the filter layer 105 to form a diffusion region, the thickness of the carrier blocking layer 104 is 1 to 2 μm, and typically the diffusion of impurities is carried out to this depth.
This means that in conventional PIN photodiodes, even taking InGaAsP as the material for the filter layer 105 and being able to epitaxially grow the filter layer 105 thickly leaves the problem that it is difficult to diffuse impurities deeply and with good control. It should be noted that if the photodiode is a mesa PIN photodiode, in which the carrier blocking layer 104 and the filter layer 105 are doped so as to be p-type in advance and the element is separated by etching, it is no longer necessary to partially diffuse the impurities in an island-shape, but mesa PIN photodiodes, particularly those that are InGaAs/InP, have a large dark current and are less reliable than planar PIN photodiodes fabricated by impurity diffusion.